string_decoder: don't cache Buffer.isEncoding

Some modules are monkey-patching Buffer.isEncoding, so without this
they cannot do that.

Fixes: https://github.com/iojs/io.js/issues/1547
PR-URL: https://github.com/iojs/io.js/pull/1548
PORT-PR-URL: https://github.com/iojs/io.js/pull/1560
PORT-FROM: v2.x / 0fa6c4a6


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>